Macro that detects and circles invalid data automatically (Data Validation)

I have a worksheet with many dependant dropdowns that are liable to having invalid data if the initial drop down is altered. I would like a macro that would automatically run the "circle invalid data" command everytime a new value is entered in a cell within a specified range (D8:T800)

It sounds fairly straightforward but i am not sure how to do this.

Question - as this macro will run every single time a cell is amended would this macro slow down the worksheet?

Many thanks,
Dan

Anyone able to help please?